In certain kinds of data communication systems using radio links, it is usual to use frequency-shift modulation in signal modulating processes. This enables standard components to be used to a large extent on the receiver side, for instance in the final stage of the intermediate frequency part and in the FM detector. The intermediate frequency amplifier will then consist of an amplitude limiting amplifier which delivers an MF signal of constant amplitude and of the square-wave type. The most common type of detector is of the analogue type, for instance, a quotient detector which produces an analogue detected signal. In the majority of mobile radio systems, digital signal processing is used to detect and to correct errors. It is known to use an A/D-converter connected to the output of the FM-detector in order to obtain a digital signal for signal processing purposes. In the case of a mobile data communication system, for instance, of the MOBITEX type (registered trademark), problems occur with regard to detector temperature drift and detector zero point errors.
It is known, for instance in U.S. Pat. No. 3,548,328, U.S. Pat. No. 3,600,680, U.S. Pat. No. 3,670,250, U.S. Pat. No. 4,236,110 or U.S. Pat. No. 4,409,984 to determine in an FM receiver and a frequency meter the frequency of a square wave of frequency f with the aid of a digital period counter which is clocked with a reference oscillator with a high degree of accuracy. The reference oscillator having frequency f.sub.r feeds a counter which counts-in reference pulses f.sub.r over a maximum measuring time T. The maximum measuring time T is determined by the information theoretical consideration (the sampling theorem). In the studied system, the requisite sampling frequency is 32 kHz, which gives a maximum measuring time of 1/32.10.sup.-3s.
When the center frequency of the FM signal is f.sub.nom, it is possible to count in approximately f.sub.nom T periods of the signal f during the measuring time T. If a fixed number of N.sub.1 =f.T periods of the current intermediate frequency f is now allowed to gate reference frequency pulse forwards to the counter, the counter will counting N.sub.2 periods of the reference frequency f.sub.r. The period time for N.sub.1 periods of f can then be determined with a normal counter uncertainty of .+-.1 period of the reference frequency f.sub.r. The number of pulses is a non-linear function of the detected frequency and therefore requires a linearising operation in order for the output signal to be a linear function of the frequency. Such a conversion can be effected, for instance, through the use of tables or with the aid of an appropriate arithmetical operation, in a manner known per se. Similarly, the modulation frequency can be obtained through the use of a simple, known operation.
The error in the time period will be EQU .epsilon.=.+-.1/f.sub.r.1/N.sub.1 =.+-.1/f.sub.r.1/f.sub.nom.1/T and the relative error will be .epsilon.=.+-.1/f.sub.r.T (1)
According to one current example, in which the reference frequency obtained from a clock generator common to the unit, f.sub.r was equal to 12.8.10.sup.6 Hz and T was equal to 10.sup.-3 /32 s.
This gave a relative error of .epsilon..sub.r =2.5.10.sup.-3. Consequently, at an intermediate frequency of 450.10.sup.3 Hz the uncertainty in the determination of the frequency f is EQU .epsilon.=450.10.sup.3.2.5.10.sup.-3 ( 2)
Consequently, it is scarcely possible to determine the frequency with an accuracy greater than .+-.1125 Hz when using purely digital methods at the intermediate frequency of 450 kHz normally used.